RF amplifiers are devices that attempt to replicate an RF signal present at an input, producing an output signal with a much higher power level. The increase in power from the input to output is referred to as the ‘gain’ of the amplifier. When the gain is constant across the dynamic range of the input signal, the amplifier is said to be ‘linear’. Amplifiers have limited capacity in terms of power delivered because of gain and phase variances, particularly saturation at high power, which makes all practical amplifiers nonlinear when the input power level varies. The ratio of the distortion power generated relative to the signal power delivered is a measure of the non-linearity of the amplifier.
In RF communication systems, the maximum allowable non-linearity of the amplifier is specified by government agencies such as the FCC or the ITU. Because amplifiers are inherently nonlinear when operating near saturation, the linearity requirements often become the limitation on rated power delivering capability. In general, when operating near saturation, the linearity of the amplifier degrades rapidly because the incremental signal power delivered by an amplifier is proportionally less than the incremental distortion power generated.
Various compensation approaches are conventionally applied to reduce the distortion at the output of the system, which in turn increases the rated power delivering capability. The preferred approach is feed forward compensation. In feed forward RF power amplifiers an error amplifier is employed to amplify main amplifier distortion components which are then combined out of phase with the main amplifier output to cancel the main amplifier distortion component. In general, feed forward compensation provides the power capability of the main amplifier and the linearity of the error amplifier.
Feed forward linearization of an amplifier is based on the matching of the gain and phase of parallel RF paths to either cancel the carrier (input) signal (loop 1) or to cancel the distortion (loop 2). The carrier cancellation is usually referred to as the ‘loop 1 error’, which is an estimate of the distortion of the main amplifier path. The distortion cancellation occurs within loop 2, and uses the loop 1 error to cancel the distortion of the main amplifier. The matching of the gain and phase in the respective loops is referred to as ‘loop alignment control’. When the alignment of loop 2 is correct, the distortion at the output is minimized, making the entire feed forward system more linear than the main amplifier alone. When the alignment of loop 1 is correct, the power through the error amplifier (which amplifies the loop 1 error) is limited. In most cases, the loop 1 alignment must be completed before the error amplifier of loop 2 is enabled. This ensures that the error amplifier is not over-driven, a condition that could produce unwanted distortion or device damage.
There have been numerous prior approaches to feed forward linearization, the earliest dating to the 1920's. In earlier approaches, the alignment settings were static, with fixed settings for gain and phase, optimized for nominal operating conditions. Later adaptive methods were applied where the misalignments of the loops were measured internally and used for subsequent alignment adjustments. Adaptive feed forward control systems can provide for compensation of dynamically changing parameters, such as temperature and DC supply variations, which affect amplifier performance. Generally, it is desirable to have the feed forward amplifier control system adapt to the optimal settings as fast as possible to minimize the amount of time the amplifier operates at a less than optimal setting.
Adaptive feed forward control methods employ alignment control algorithms to adjust the alignment settings (gain and phase) from any initial setting to that which results in the best measured alignment. Prior alignment control algorithms rely on either the “steepest descent” or the “coordinate descent” algorithms. The steepest descent algorithm adjusts the alignment settings in a direction of the gradient within the 2D gain-phase space. Dithering the alignment in orthogonal directions and measuring the changes in measured misalignment provides an estimate of the gradient. The coordinate descent algorithm performs two separate 1D searches along pre-defined orthogonal directions (usually the gain and phase axes). The alignments are dithered to determine which direction along the respective coordinates reduces measured misalignment. Both these approaches have disadvantages in practical systems which employ control processors with limited processing power and where fast loop alignment is desired. As a result the desired fast and accurate loop convergence has not been achieved in practical adaptive feed forward systems.
Accordingly, a need presently exists for a system and method for rapid loop alignment control in a feed forward amplifier system which avoids the above noted limitations of the prior art.